1. Field of the Invention
The present invention relates to the field of micromachined circuit elements.
2. Prior Art
Micromachined circuit elements are currently the subject of increasing interest because of the advantages such circuit elements have in various applications. By way of example, U.S. Pat. No. 5,491,604 discloses Q-controlled microresonators and tunable electronic filters using such resonators. U.S. Pat. No. 5,537,083 discloses micromechanical signal processors, including micromechanical filters asserted to achieve high signal-to-noise ratios and quality factors on the order of 50,000 in a vacuum. U.S. Pat. No. 4,674,180 discloses a Method of Making a Micromechanical Electric Shunt, and U.S. Pat. No. 5,638,946 discloses a micromechanical switch with insulated switch contact.
In these and other micromechanical devices, mechanical motion is normally caused by electrostatic attraction between two conductive plates (one of which frequently is stationary) having different voltages thereon. In general, the force of attraction between two parallel plates is proportional to the area of the plates times the square of the voltage difference between the plates, divided by the square of the separation between the plates. For devices such as micromachined switches, the full supply voltage difference may be used to actuate the switch. There is, however, a continuing desire to operate integrated circuits at lower and lower supply voltages, which grossly reduces the force available for operation of a switch. For this reason, it is now common to operate micromachined switches from a voltage which is higher than the integrated circuit power supply.
In the case of micromachined oscillatory devices such as resonators, tuned filters and the like, the mechanical force initiating and maintaining the mechanical oscillation of the micromachined devices is normally generated by superimposing an AC voltage on a DC voltage difference between the two plates. One way to achieve this is to provide the drive at the desired drive frequency by alternately switching the full supply voltage across the plates and then shorting the two plates together at the desired drive frequency. This will generate a square wave of force on the resonant member, oscillating between zero force and the maximum force available from the power supply voltage. Alternatively, the oscillatory element may be biased at the power supply voltage through a high resistance, and a voltage switched between the power supply voltage and ground at the desired frequency capacitively coupled thereto. This can potentially double the driving force, though in either case, only a fraction represents the sinusoidal force at the desired frequency, the remainder being harmonics thereof.